Infrastructure Technology Institute
All bridges depend upon their substructure - piers, columns, and abutments - to safely transfer loads to the ground. Reinforced concrete, the most commonly used substructure material, is vulnerable to damage from corrosion, impacts, overloads, and earthquakes, to name a few damage mechanisms. One common repair for concrete columns that have suffered limited damage or need increased load capacity is a column wrap, in which the outer surface of the substructure element is wrapped with layers of high strength fibers and adhesive to form a composite shell.
Composite column wrapping can increase the flexural ductility (ability to bend without breaking) and increase the shear strength (ability to resist side loading) of concrete columns. The column wrap accomplishes this by radially confining the concrete core, thereby taking advantage concrete’s innately high compressive strength. As long as the column stays column-shaped, it remains strong. This confinement will also increase the column’s resistance to buckling even after a plastic hinge has formed in the wrapped region. Column wrapping also improves blast resistance and provide additional corrosion protection. However, proper installation of a column wrap is critical - improperly installed wrapping can appear acceptable under visual inspection, but not provide any benefits.
Northwestern University’s Center for Quality Engineering & Failure Prevention and ITI jointly developed a pulsed themography imaging system for non-destructive testing of installed composite column wrapping. This technique uses portable high intensity flash lamps to briefly induce localized heating of the wrapping surface. The elevated surface temperature of the wrap from the flash drops as the heat is conducted through the wrap and into the underlying column. This changing temperature pattern on the surface is then recorded for several seconds with a sensitive infrared video camera. Poorly bonded areas will retain heat longer than those securely bonded to the concrete because the gap between layers acts as a thermal insulator. The result is an easily interpreted video showing problem areas staying brighter longer than others for a few seconds as the heat from the initial flash dissipates. Pulsed thermography can be used as both a long term inspection tool and a quality control check for newly installed wraps.
Northwestern researchers and IDOT engineers recently field tested the new system during an inspection of the approach structure to the I-57 Mississippi River Bridge located in Cairo, Illinois. This bridge has been extensively reinforced with composite column wrappings for seismic protection. In Cairo, the pulsed thermography inspection system proved capable of detecting and quantifying disbonds between the concrete/wrapping interface and also between layers within wrappings. Prior to this demonstration, the best method to detect disbonds was by visual inspection and manual tapping to find “dead” sounding areas. The thermography system provided results quickly, which is critical for minimizing inspection costs on large structures. Pulsed thermography techniques also generate a video record of testing, which is important for inspection records and long-term monitoring of these composite wraps.